1. Field of the Invention
This invention relates to a servo track writer for a head disk assembly and a method of making the head disk assembly using the servo track writer.
2. Description of the Prior Art and Related Information
With reference to FIGS. 1A and 1B, a prior art servo track writer 100 is shown which includes a supporting block 102 for supporting a plurality of head disk assemblies (not shown), e.g. four head disk assemblies, and a platform ("bell housing") 32. In the course of making a head disk assembly (HDA), servo track writer 100 is used to write servo information on a recording surface of a disk within the HDA. Supporting block 102 includes apertures 104. Platform 32 includes a base portion 30 and a plurality of columns ("necks") 20. Servo track writer 100 further includes a plurality of servo writer positioning arms 10, each positioning arm extending through a respective aperture 104 and associated with a respective HDA.
A typical head disk assembly includes an enclosure having a base and a cover, a spindle motor attached to the base, a disk mounted on the spindle motor, and a head stack assembly coupled to the base. The head stack assembly includes a plurality of actuator arms including a bottom arm and a head gimbal assembly attached to each actuator arm. Each head gimbal assembly includes a load beam and a head coupled to the load beam.
In operation, four head disk assemblies are secured on supporting block 102 in order to write servo information simultaneously on each disk of the head disk assemblies. Control circuitry (not shown) of servo writer 100 controls the spin rate of each disk via a spindle motor (not shown) and each positioning arm 10 is moved simultaneously in a counter-clockwise direction through an opening in the enclosure such as a side opening defined by the side walls of the base and cover. Each head stack assembly is moved towards an area of the disk proximate an outer diameter of the disk such that a bottom actuator arm 50 is biased against the positioning arm as shown in FIG. 1C. While bottom actuator arm 50 is biased against positioning arm 10, the positioning arm is moved in the counter-clockwise direction such that writing of servo information can occur on each recording surface of each disk ("servo writing process").
During the servo writing process, the disks are rotated at a relatively high speed, such as 5400 revolutions per minute (rpm), which generates a large degree of air turbulence inside an HDA. Such air turbulence causes both the positioning arm 10 and the actuator arms to vibrate since bottom actuator arm 50 is abutting positioning arm 10.
The vibrations lead to several problems during the servo writing process. First, the vibrations of positioning arm 10 may cause platform 32 to vibrate at a frequency at or near a resonant frequency of the platform which may adversely affect the servo control of positioning arm 10. Another problem resulting from the vibrations is that because the actuator arms are vibrating during the servo writing process, the servo information may not be written as concentric tracks on a recording surface. Rather, the written tracks may have high frequency sinusoidal vibration displacements superimposed on the track profile, resulting in a written-in or repeatable run-out (RRO) of the tracks. Such run-out results in a decrease in HDA yields and/or will limit the number of tracks that can be written on the recording surface decreasing the storage capacity.
With reference to FIG. 2, the vibrations of a prior art servo writer positioning arm as a function of frequency are shown. The vibrations were measured with an accelerometer mounted on a back side of the positioning arm such as the one shown in FIG. 1C. Since the positioning arm is abutting an actuator arm such as bottom actuator arm 50 shown in FIG. 1C, the vibrations of the actuator arm are substantially the same. As shown, at particular frequencies, the magnitude of the vibrations induced in the actuator arm is relatively large which leads to the problems described above.